Catalyst and dehydrogenation process

ABSTRACT

A catalyst containing alkali metal, chromium, iron, lead, phosphorus and oxygen, and a process for the preparation of unsaturated lower aliphatic acids and esters of such acids by catalytic oxidative dehydrogenation of the corresponding saturated acids and esters using the catalyst.

BACKGROUND OF THE INVENTION

Unsaturated lower aliphatic acids and esters, and particularlymethacrylic acid and its esters, are used in the preparation of polymerswhich find wide commercial applicability.

The preparation of such unsaturated lower aliphatic acids and esters istypically carried out by the catalytic oxidative dehydrogenation of thecorresponding saturated acids and esters. A wide variety of catalystshas been proposed for such reactions. However, the catalysts previouslyproposed share one or more deficiencies. Earlier catalysts, for example,frequently result in an undesirably low conversion in the catalyticprocess or low selectivity for the desired unsaturated acid product.Others exhibit short catalytic life or higher cost than would becommercially acceptable. Accordingly, continued effort has been directedtoward the development of better catalysts for this oxidativedehydrogenation reaction.

SUMMARY OF THE INVENTION

The present invention provides an improved catalyst that results in highselectivity and yield in the oxidative dehydrogenation of saturatedlower aliphatic acids and esters to the corresponding unsaturated acidsand esters.

Specifically, the instant invention provides a catalyst of the generalformula:

    Me.sub.a Cr.sub.b FePb.sub.c P.sub.d O.sub.e

wherein Me is an alkali metal, a is about from 0.01 to 0.3, b is aboutfrom 0.05 to 0.4, c is about from 0.6 to 1.0, d is about from 1.0 to 6.0and e is sufficient to provide oxygen to satisfy the oxidative states ofthe remaining components.

The instant invention further provides an improvement in the process forthe oxidative dehydrogenation of lower aliphatic saturated acids andesters of such acids by heating in the presence of a catalyst to yieldthe corresponding unsaturated lower aliphatic acids and esters, whichimprovement comprises using a catalyst of the above formula.

DESCRIPTION OF THE INVENTION

The present invention is based on the discovery of a phosphate catalystcontaining alkali metal, chromium, iron, and lead as shown by the aboveformula which is particularly well suited for the dehydrogenationreaction used to produce unsaturated acids and esters such asmethacrylic acid.

The catalyst of the present invention can be readily prepared by firstdissolving salts of the required metals in water to form a solution.While any water-soluble salt of the metal can be used, such as nitratesand chlorides, nitrates of the required metals have been found to beparticularly satisfactory. For example, the iron component of thecatalyst can be provided from ferric nitrate enneahydrate; chromium canbe provided from chromium nitrate enneahydrate; lead can be providedfrom lead nitrate and the alkali metal from its corresponding nitrate.

The phosphorus in the catalytic compound can be conveniently provided byadding, to the metal salt solution, an amount of phosphoric acid ordibasic ammonium hydrogen phosphate necessary to provide the desiredamount of phosphorus in the final product.

After preparation of an aqueous solution containing the alkali metal,chromium, iron, lead, and phosphorus components of the catalyst, thesolution can be precipitated by the addition of concentrated ammoniumhydroxide to form a more basic solution, having a pH, for example, ofabout 5.0. The resulting precipitated catalytic components can then befiltered, washed and calcined with oxygen to complete the reaction tothe desired catalytic compound. Typically, the composition is calcinedat about 500° C. for two hours or more.

The alkali metal used in the preparation of the catalysts can beselected from one or more of lithium, sodium, potassium, rubidium andcesium. However, of these, sodium, potassium and cesium are preferredfor their contribution to high selectivity, and sodium and cesium areparticularly preferred.

The present catalysts can be used in conjunction with various supportsconventionally used for catalysts of this type, including pumice,Kieselguhr, zirconium dioxide and colloidal silica. However, collodialsilica has been found to be particularly satisfactory in the presentinvention, providing increased conversion. While this effect is notfully understood, it is believed to be a function of the increasedsurface area provided by the silica support. The colloidal silica can beconveniently incorporated into the catalyst by adding the colloidalsilica to the solution of catalyst components before precipitation andcalcination.

The catalytic dehydrogenation for which the catalysts are particularlywell suited involves contacting saturated lower aliphatic acids andesters with oxygen in the presence of the catalyst at elevatedtemperatures. Conventional fixed tube or fluid bed reactors can be usedfor this conversion. The general conversion process is well known in theart and described, for example, in U.S. Pat. No. 3,652,654. The acids oresters used in the reaction have the general formula ##STR1## whereineach of R₁, R₂, R₃ and R₄ is selected from hydrogen and lower alkylgroups containing 1-4 carbon atoms. In the instant process, theconversion temperatures used are generally about from 250° to 500° C.,and preferably about from 325° to 400° C. The pressure, for ease ofoperation, is generally maintained at atmospheric pressure or somewhatabove. However, pressures within the range of 1-10 atmospheres can beused effectively.

The instant process is particularly well suited to the conversion ofisobutyric acid to methacrylic acid. The use of the present catalysts inthis process results in good conversions with excellent selectivity forthe desired methacrylic acid as opposed to acetone or other by-products.

The instant invention is further illustrated by the following specificexamples, in which parts and percentages are by weight unless otherwiseindicated.

EXAMPLES

In Examples 1-14 and Comparative Examples A-H, the catalyticcompositions were prepared by first forming a solution of ferric nitrateenneahydrate, chromium nitrate enneahydrate, alkali metal nitrate andlead nitrate in distilled water at 50° C., each of the nitrates beingpresent in quantities that will result in the desired atomic ratio ofthe metal in the final product. To this solution was added 85%phosphoric acid, also in quantities sufficient to provide the requiredamount of phosphorus in the final product. This solution was stirred forten minutes, and had a highly acidic character, exhibiting a pH of lessthan 1.0.

In those examples using colloidal silica as a catalyst support, thesilica was added to the solution with additional stirring for tenminutes.

The catalytic components were precipitated from solution by the additionof ammonium hydroxide in an amount sufficient to adjust the pH of thesolution to 5.0. After precipitation, the slurry was heated at 100° C.for two to three hours to assure precipitation of the silica, whenpresent. The slurry was then cooled to room temperature, filtered, andwashed three times with water. The resulting catalyst cake was dried ina vacuum oven at 140° C. for eight hours, broken into four to ten meshparticles, and calcined at 500° C. for two hours with oxygen.

The resulting catalyst was broken into particles of 8 to 10 mesh forfixed bed reactors, and 100 to 170 mesh for fluidized bed reactors.

In Examples 1-8 and Comparative Examples A-C, a fixed bed catalyticprocess was used for the conversion of isobutyric acid to methacrylicacid. The catalyst granules, prepared as described above, were packed ina 3/8 in. stainless steel fixed bed reactor. About 1.2 cc of catalysthaving an 8 to 10 mesh particle size were used. The liquid isobutyricacid was continuously vaporized and mixed with air and the mixed gaseswere preheated and fed to the reactor. The temperature was maintained byheating the reactor with a fluidized sand bath. The contact times areexpressed as the ratio of catalyst volume to total gas feed rate, andcalculated assuming ideal gas behavior. The units used are cubiccentimeters of catalyst per cubic centimeters of gas feed per second at25° C. and 760 millimeters of mercury.

In Examples 1-8 and Comparative Examples A-C, conversion and selectivityare determined based upon the total moles of isobutyric acid in the feedstream and the total moles of isobutyric acid and methacrylic acidcontained in product stream. Percent conversion is calculated bysubtracting the moles of isobutyric acid recovered from the moles ofisobutyric acid introduced into the reactor, dividing by the moles ofisobutyric acid introduced, and multiplying by 100. The percentselectivity is the moles of product formed divided by the difference inthe moles of isobutyric acid introduced into the system and the moles ofisobutyric acid recovered, multiplied by 100. The quantities of productused in these calculations is determined by gas chromographic analysisof the product stream taken from the reactor.

In Examples 7 and 8, large pore silica catalyst supports with lowsurface areas were used in place of the colloidal silica. Pumice wasused in Example 7 and Kieselguhr was used in Example 8. The catalystwere prepared as previously described, except that pumice or Kieselguhrwas added to the aqueous solution instead of the colloidal silica.

The catalytic compounds used in Comparative Examples A-C were preparedaccording to German Patent Publication No. 2438646.

In Examples 9-16, the catalysts of indicated composition were ground andscreened through a 100 mesh screen and retained on a 170 mesh screen.These catalysts were used in a fluidized bed reactor which consisted ofa 1 inch outer diameter stainless steel tube fitted with a 30 micronsintered metal disk on the bottom. A mixture of air, water andisobutyric acid in the indicated ratios was fed continuously to thebottom of the reactor, the temperature which was controled by means of afluidized sand bath.

The atomic ratios of the catalysts used, the operating conditions andthe results of the Examples are summarized in the following Tables I toIII.

                  TABLE I                                                         ______________________________________                                        OXIDATIVE DEHYDROGENATION OF ISOBUTYRIC                                       ACID FLUIDIZED BED PROCESS                                                    Catalyst Composition (Atomic Ratio)                                           Me                             Silica                                         Example Metal   Ratio   Fe  Pb  Cr   P     Support                            ______________________________________                                        1       Cs      0.1     1.0 0.9 0.25 4.7   --                                 2       Cs      0.1     1.0 0.9 0.25 4.7   3.0                                3       Na      0.01    1.0 0.8 0.11 4.2   --                                 4       Cs      0.07    1.0 0.8 0.11 4.4   2.65                               5       K       0.07    1.0 0.8 0.11 4.4   2.65                               6       Na      0.07    1.0 0.8 0.11 4.4   2.65                               7       Na      0.07    1.0 0.8 0.11 4.4   --                                 8       Na      0.07    1.0 0.8 0.11 4.4   --                                 9       Cs      0.1     1.0 0.9 0.25 4.7   --                                 10      Cs      0.1     1.0 0.9 0.25 4.7   3.0                                11      Cs      0.07    1.0 0.8 0.11 4.4   2.65                               12      K       0.07    1.0 0.8 0.11 4.4   2.65                               13      Na      0.07    1.0 0.8 0.11 4.4   2.65                               14      Na      0.07    1.0 0.8 0.11 4.4   2.65                               A       --      --      1.0 0.6 --   3.3   --                                 B       --      --      1.0 0.6 --   3.3   --                                 C       --      --      1.0 0.6 --   3.3   1.6                                D       Bi 2.0  Bi 2.0  1.0 --  --   3.0   --                                 E       Bi 2.0  Bi 2.0  1.0 --  --   3.0   --                                 F       Cs      0.66    1.0 --  --    1.84 --                                 G       --      --      1.0 0.6 --   3.3   1.6                                H       Cs      0.66    1.0 --  --    1.84 --                                 ______________________________________                                    

                  TABLE II                                                        ______________________________________                                                Reaction Conditions                                                                      Mole    Mole  Mole  Contact                                          Temp     %       %     %     Time                                   Example   ° C.                                                                            IBA     AIR   H.sub.2 O                                                                           Sec                                    ______________________________________                                        1         400°                                                                            13.5    32.5  54.0  0.22                                   2         400°                                                                            13.5    32.5  54.0  0.22                                   3         400°                                                                            13.5    32.5  54.0  0.30                                   4         400°                                                                            8.0     18.5  73.5  0.28                                   5         400°                                                                            8.0     18.5  73.5  0.30                                   6         400°                                                                            8.0     18.5  73.5  0.26                                   7         400°                                                                            8.0     18.5  73.5  0.26                                   8         400°                                                                            8.0     18.5  73.5  0.26                                   9         400°                                                                            15      22.6  61.9  1.43                                   10        399°                                                                            7.4     21.1  71.2  1.01                                   11        350°                                                                            81      18.6  73.3  2.33                                   12        376°                                                                            7.9     70.3  71.8  1.71                                   13        350°                                                                            8.04    19.2  73.1  2.90                                   14        350°                                                                            7.9     20.7  71.4  2.49                                   A         400°                                                                            13.5    32.5  54.0  0.22                                   B         400°                                                                            8.0     18.5  73.5  0.26                                   C         400°                                                                            8.0     18.5  73.5  0.30                                   D         400°                                                                            13.5    32.5  54.0  0.22                                   E         400°                                                                            8.0     18.5  73.5  0.28                                   F         400°                                                                            8.0     18.5  73.5  0.28                                   G         389°                                                                            15.5    23.0  62.0  1.70                                   H         350°                                                                            8.0     21.0  70.9  2.35                                   ______________________________________                                    

                  TABLE III                                                       ______________________________________                                        ______________________________________                                                 Conversion & Selectivity                                                      %          % Selectivity                                             Example    Conversion   MAA       Acetone                                     ______________________________________                                        1          30.6         83.9      15.7                                        2          22.0         84.8      14.5                                        3          34.8         80.0      13.0                                        4          51.8         80.8      12.4                                        5          57.8         83.4      13.8                                        6          55.0         84.5      11.8                                        7           6.9         81.6      19.1                                        8          30.0         82.0      14.6                                        9          57.5         66.0      15.2                                        10         49.5         79.4      10.3                                        11         35.0         78.1      16.6                                        12         53.1         77.6      15.4                                        13         62.8         85.0      13.0                                        14         39.5         85.5      10.4                                        A          29.0         70.0      14.6                                        B          33.3         63.1      12.5                                        C          36.7         51.7      13.5                                        D          34.8         71.9      28.2                                        E          33.5         70.3      27.1                                        F          36.8         44.6      13.8                                        G          58.2         64.0      10.6                                        H          39.4         62.2      12.5                                        ______________________________________                                    

We claim:
 1. In the process for the oxidative dehydrogenation of asaturated compound of the general formula: ##STR2## wherein each of R₁,R₂, R₃ and R₄ is selected from hydrogen and lower alkyl groupscontaining 1-4 carbon atoms, by heating the compound in the presence ofa catalyst and oxygen to yield the corresponding unsaturated compound,the improvement which comprises heating the compound in the presence ofa catalyst of the general formula:

    Me.sub.a Cr.sub.b FePb.sub.c P.sub.d O.sub.e

wherein Me is an alkali metal selected from the group consisting oflithium, sodium, potassium, rubidium, and cesium, a is about from 0.01to 0.3, b is about from 0.05 to 0.4, c is about from 0.6 to 1.0, d isabout from 1.0 to 6.0 and e is sufficient to provide oxygen to satisfythe oxidative states of the remaining components.
 2. A process of claim1 wherein the alkali metal of the catalyst is potassium, sodium orcesium.
 3. A process of claim 1 wherein the catalyst further comprises asupport of colloidal silica.
 4. A process of claim 1 wherein thesaturated compound is isobutyric acid.